1. Field of the Invention
This invention relates generally to radio frequency (rf) linear accelerator (linac). Specifically, this invention relates to rf power system for generating high voltages on electrodes to accelerate ion beams.
2. Description of the Prior Art
Strong rf electromagnetic fields, bounded by resonant cavities, are commonly used in particle accelerator systems to accelerate charged particle beams. Gustav Ising proposed the first accelerator that used time-dependent fields in 1924.sup.i. The concept proposed by Ising was not tested at that time until Rolf Wideroe conceived and demonstrated experimentally the first RF linear accelerator in 1927.sup.ii. The linac built by Wideroe was the forerunner of all modern RF accelerators. Wideroe's concept was to apply a time-alternating voltage to a sequence of drift tubes whose lengths increased with increasing particle velocity, so that the particles would arrive in every gap at the right time to be accelerated.
In Wideroe's experiment, an rf voltage of 25 kV from a 1-MHz oscillator was applied to a single drift tube between two grounded electrodes, and a beam of singly charged potassium ions gained the maximum energy in each gap. A final beam-energy of 50 keV was measured, which was twice that obtainable from a single application of the applied voltage. This was also the first accelerator that had ground potential at both the entrance and the exit ends, and was still able to deliver a net energy gain to the beam, using the electric fields within. The experiment established the principle that unlike an electrostatic accelerator, the voltage gain of an rf accelerator could exceed the maximum applied voltage.
Wideroe's experiment had great influence on modern linacs. In 1931, Sloan and Lawrence built a Wideroe-type linac with 30 drift tubes and by applying 42 kV at a frequency of 10 MHz, they accelerated mercury ions to an energy of 1.26 MeV.sup.iii. Luis Alvarez and co-workers proposed.sup.iv and built.sup.v a 1-m-diameter 12-m-length drift-tube linac (DTL) at a frequency of 200 MHz, which accelerated protons from 4 MeV to 32 MeV. Kapchinskly and Tepliakov first presented the principles of operation of a radio-frequency quadrupole linac (RFQ).sup.vi, which had four electrodes with modulated shapes to produce transverse and longitudinal electric fields to achieve both focusing and acceleration of charged particle beams.
Linac technology has been used for accelerating ions up to MeV range for ion implantation application since 1980.sup.vii. The recent development of linac is focused on improvement of linac power efficiency to reduce build cost and power consumption.sup.viii,ix. Except the RFQ linacs, which have very small mass and energy range, a linac used in ion implantation is very similar to Wideroe's linac and consists of individual rf electrodes that can accelerate ion beams up to MeV energies (FIG. 1). Each electrode can be driven to a very high voltage (40.about.100 kV) by a resonant LC circuit The resonant circuit including a coil, electrode, and enclosure wall, is referenced to as a resonator. The merit of the individual resonator configuration is that each resonator can be independently tuned and the linac has more flexibility for accelerating ion species with different masses and charge states to achieve the desired ion energy.
However, each resonator system, including the resonator, rf amplifier, and tuning electronics is very expensive. A resonator also consumes several kilowatts of electric power. Most of the power is dissipated as heat inside the resonator and less than 5% of the electric power can be used to accelerate the ion beams.
Increasing power efficiency of a resonator can reduce power consumption and the cost of rf amplifiers if the final ion energy is kept unchanged. For the same power consumption per resonator, higher power efficiency also means higher output voltage on the rf electrode, resulting higher energy gain per resonator. The number of resonators can be reduced for the same final ion energy. Glavish claimed that his single electrode resonator could deliver 95 kV rf voltage on the electrode with 2.6 kW rf input power.sup.8. Since there were two rf acceleration gaps in this single-electrode resonator system, the maximum energy gain per input power square root (since rf voltage square is proportional to rf power) is 118 keV/(kW).sup.1/2 for a singly-charged ion. A single-electrode resonator in Eaton's most recent high-energy rf linac could deliver 80 kV rf voltage with 3.0 kW input power. The maximum energy gain per input power would be 92 kV/(kW).sup.1/2.
Increasing the number of acceleration gaps is another way to increase energy gain per input power. Fujisawa's triple-gap rf resonator could obtain energy gain of 216 kV at input power of 5 kW.sup.xi. The energy gain per input power was 97 kV/(kW).sup.1/2.
The above mentioned double-gap and triple-gap resonators have energy gain per input power around 100 kV/(kW).sup.1/2 proven by beam tests while Glavish's resonator has so far no supporting ion beam experimental data for proving the claim of a high efficiency. Conventional techniques of ion beam acceleration according to above brief survey of the prior art techniques illustrates that there are still limitations now encountered by those involved in design and manufacturing a linear ion accelerator. These prior art techniques do not provide a viable solution to enable a person of ordinary skill in the art to overcome these limitations. Therefore, it is necessary to use a new approach to increase the acceleration efficiency over the limitation of the state of the art The present invention of double-electrode resonator demonstrates a new concept for ion beam acceleration with the purposes and objects of providing much higher acceleration efficiency.